ear

Chapter 15: Special Senses - Ear

Introduction

• Course: BIOL 2401
• Lecture: 21 from CTC A&P Program
• Text Pages: 574-588

Overview of the Ear

• The ear is subdivided into three major parts:
    - External (outer) ear: Responsible for hearing only.
    - Middle ear (tympanic cavity): Responsible for hearing only.
    - Internal (inner) ear: Responsible for both hearing and equilibrium; contains receptors for hearing and balance that respond to separate stimuli and are activated independently.

External Ear

• Composition of the External Ear:
    - Auricle (pinna):
      - Shell-shaped structure surrounding the ear canal.
      - Funnel sound waves into the auditory canal.
    - Helix: Cartilaginous rim of the auricle.
    - Lobule: Fleshy earlobe.
    - External Acoustic Meatus (auditory canal):
      - Short, curved tube lined with skin that has hairs, sebaceous glands, and ceruminous (earwax) glands.
      - Transmits sound waves to the tympanic membrane (eardrum).

• Tympanic Membrane (Eardrum):
    - Acts as a boundary between the external and middle ear.
    - Made of thin, translucent connective tissue membrane.
    - Vibrates in response to sound, transferring sound energy to the bones of the middle ear.

Middle Ear (Tympanic Cavity)

• Description:
    - A small, air-filled cavity lined with mucosa, located in the temporal bone.
    - Flanked laterally by the eardrum and medially by a bony wall containing oval and round membranous windows.

• Pharyngotympanic (auditory) tube:
    - Connects the middle ear to the nasopharynx.
    - Formerly known as Eustachian tube.
    - Usually flattened, can be opened by yawning or swallowing to equalize pressure between middle ear and external air.
    - Tympanic membrane cannot vibrate efficiently if the pressures on both sides are unequal.

• Auditory Ossicles:
    - Three small bones in the tympanic cavity, named after their shape:
      - Malleus: The “hammer” secured to the eardrum.
      - Incus: The “anvil.”
      - Stapes: The “stirrup,” base fits into the oval window, effectively amplifying vibrations.

Clinical Considerations

• Otitis Media:
    - Inflammation of the middle ear, commonly seen in children with sore throats.
    - Particularly prevalent in those with shorter, more horizontal pharyngotympanic tubes.
    - Most frequent cause of hearing loss in children.
    - Acute forms can cause the eardrum to bulge and become inflamed, most cases respond to antibiotics.

Internal Ear (Labyrinth)

• Location:
    - Situated in the temporal bone, behind the eye socket.
• Major Divisions:
    - Bony Labyrinth: A system of channels and cavities through the bone, divided into three regions: vestibule, semicircular canals, and cochlea.
      - Filled with perilymph fluid, similar to cerebrospinal fluid (CSF).
    - Membranous Labyrinth: A series of membranous sacs and ducts contained within the bony labyrinth; filled with potassium-rich endolymph.

Vestibule

• Description:
    - Egg-shaped central cavity of the bony labyrinth.
    - Contains two membranous sacs:
      - Saccule: Continuous with cochlear duct.
      - Utricle: Continuous with semicircular canals.
    - Houses equilibrium receptor regions (maculae) that respond to gravity and change in head position.

Semicircular Canals

• Description:
    - Three canals oriented in three different spatial planes: anterior, lateral, and posterior.
    - Anterior and posterior canals are at right angles to each other; the lateral canal is horizontal.
    - Membranous semicircular ducts line each canal and communicate with the utricle.
    - Ampulla: The enlarged area of each canal that houses the equilibrium receptor region called the crista ampullaris, which responds to angular (rotational) movements of the head.

Cochlea

• Description:
    - A small, spiral, conical bony chamber that extends from the vestibule.
    - Coils around a bony pillar known as the modiolus.
    - Contains the cochlear duct, housing the spiral organ (organ of Corti), which ends at the cochlear apex.

• Chamber Separation:
    - Scala Vestibuli: Abuts the oval window and contains perilymph.
    - Scala Media (Cochlear Duct): Contains endolymph.
    - Scala Tympani: Terminates at the round window and contains perilymph.
    - Scala tympani and scala vestibuli are continuous at the helicotrema (apex).

• Membranes:
    - Vestibular Membrane: The roof of the cochlear duct, separating the scala media from scala vestibuli.
    - Basilar Membrane: Forms the floor of the cochlear duct, supporting the spiral organ, which contains functionally arranged hair cells.

Sound Detection and Hearing

• Definition of Hearing:
    - The reception of air sound waves converted into fluid waves, stimulating mechanosensitive cochlear hair cells that send impulses to the brain for interpretation.

Properties of Sound

• Sound Characteristics:
    - Sound is a pressure disturbance (alternating high and low pressure).
    - Represented as an S-shaped curve or sine wave, with compressions illustrated as crests and rarefactions as troughs.

Frequency and Amplitude

• Frequency:
    - The number of waves passing a given point in a given time.
    - Pure tone has repeating crests and troughs.
    - Wavelength is the distance between two consecutive crests; shorter wavelength = higher frequency.

• Human Hearing Range:
    - 20–20,000 hertz (Hz), with sensitivity best between 1500 and 4000 Hz.
    - Pitch: The perception of different frequencies; higher frequency results in higher pitch.
    - Quality: The characteristic of sounds; most sounds are mixtures of different frequencies.

• Amplitude:
    - Height of crests; perceived as loudness (subjective interpretation of sound intensity).
    - Measured in decibels (dB), with a normal range of 0–120 dB.
    - Examples:
      - Normal conversation: ~50 dB.
      - Threshold of pain: 120 dB.
      - Prolonged exposure above 90 dB can cause severe hearing loss.

Sound Transmission to Internal Ear

• Pathway of Sound:

1. Tympanic Membrane:
      - Sound waves vibrate the tympanic membrane, which then causes vibrations to occur at higher intensities.
2. Auditory Ossicles:
      - Transfer vibration to the oval window; tympanic membrane is approximately 20 times larger than the oval window, amplifying the vibrations.
3. Motion in Scala Vestibuli:
      - The stapes rocks back and forth on the oval window, initiating wave motion in perilymph, which continues to the round window.
4. Sound Pathways:
      - Waves travel through helicotrema for low frequencies and via basilar membrane for hearing range frequencies.

Resonance of the Basilar Membrane

• Definition of Resonance: The movement of different areas of the basilar membrane in response to a specific frequency.
• Fibers change along the length of the basilar membrane:
    - Fibers near the oval window are short and stiff, resonating with high-frequency waves.
    - Fibers near the cochlear apex are longer and floppier, resonating with lower-frequency waves.

Sound Transduction

• Excitation of Inner Hair Cells:
    - Movement of the basilar membrane bends the hairs of inner hair cells, leading to electrical impulses transmitted to the brain.
    - Inner hair cells have microvilli with many stereocilia that bend and open ion channels due to tip links being pulled, allowing K+ and Ca2+ ions to enter, producing a receptor potential that leads to neurotransmitter release (glutamate).
    - This can result in action potentials in afferent neurons of the cochlear nerve.

• Role of Outer Hair Cells:
    - Effector neurons that change stiffness of the basilar membrane, which increases responsiveness of inner hair cells to sound by amplifying motion and provides protection from loud sounds.

Auditory Pathways to Brain

• Neural impulses from cochlear bipolar cells reach the auditory cortex through a pathway:

1. Spiral ganglion
2. Cochlear nuclei (medulla)
3. Superior olivary nucleus (pons-medulla)
4. Lateral lemniscus (tract)
5. Inferior colliculus (midbrain auditory reflex center)
6. Medial geniculate nucleus (thalamus)
7. Primary auditory cortex

• Some fibers cross while others do not, allowing input from both ears to reach both auditory cortices.

Auditory Processing

1. Perception of Pitch: Impulses from hair cells at specific positions along the basilar membrane interpreted as distinct pitches.
2. Detection of Loudness: Determined by an increase in the frequency of action potentials corresponding to larger deflections experienced by hair cells.
3. Localization of Sound: Depends on the relative intensity and timing of sound waves reaching each ear.

Homeostatic Imbalances of Hearing

• Types of Deafness:
    - Conduction Deafness: Blockage of sound conduction to internal ear fluids, caused by conditions like impacted earwax, perforated eardrum, otitis media, or otosclerosis of the ossicles.
    - Sensorineural Deafness: Damage to neural structures from cochlear hair cells to auditory cortical cells, typically from gradual hair cell loss.

• Research:
    - Sensorineural deafness research involves promoting differentiation in supporting cells to replace hair cells.
    - Cochlear implants convert sound energy into electrical signals, effective for congenital or age-related cochlear damage, and help deaf children learn to speak.

Maintenance of Equilibrium

• Definition of Equilibrium: Response to various head movements reliant on input from the inner ear, eyes, and stretch receptors.
• Vestibular Apparatus: Contains equilibrium receptors in semicircular canals and vestibule.
    1. Vestibular receptors monitor static equilibrium.
    2. Semicircular canal receptors monitor dynamic equilibrium.

The Maculae

• Function of Maculae:
    - Sensory organs that monitor static equilibrium, with one organ located in each wall of the saccule and utricle, responding to gravity and head position.
    - Important for posture control and respond to linear acceleration forces but not rotation.

• Anatomy of a Macula:
    - Composed of a flat epithelium patch containing hair cells with supporting cells.
    - Hair cells possess stereocilia and a special true stereocilium called kinocilium, embedded in an otolith membrane that is weighted by otoliths (tiny CaCO3 stones) to increase inertia.

• Utricle and Saccule Functionality:
    - Utricle maculae respond to horizontal changes, while saccule maculae respond to vertical movements, such as acceleration in elevators.

The Cristae Ampullares

• Function of Cristae Ampullares:
    - Responsible for detecting rotational acceleration and major stimuli are rotational movements like twirling.
    - Each crista contains supporting cells and hair cells extending into an ampullary cupula.
• Activation of Hair Cells:
    - Movement of endolymph due to inertia causes bending of hair cells, leading to depolarization or hyperpolarization based on the direction of bending.

Equilibrium Pathway to the Brain

• Processing:
    - Equilibrium signals are transmitted to reflex centers in the brainstem for quick responses to maintain balance.
    - Information inputs originate from vestibular receptors, visual receptors, and somatic receptors.

Clinical Homeostatic Imbalances

• Motion Sickness:
    - Occurs when sensory inputs are mismatched; conflicting information leads to symptoms such as excessive salivation, pallor, rapid breathing, and profuse sweating.
    - Treatment options include antimotion drugs like meclizine and scopolamine.